1. Field of the Invention
This invention relates to optical detectors. Optical detectors are used in digital optical fiber communications to receive and then convert incident optical signals into corresponding electrical signals which are then amplified and processed by electronic means.
2. Related Art
High input impedance preamplifiers are commonly used in preference to low impedance or transimpedance configurations when the sensitivity of the receiver is of primary importance because of their lower thermal noise. In high impedance preamplifiers the value of the input bias resistance, used to bias the input transistor, is made arbitrarily high to reduce its thermal noise.
There are, however, three disadvantages associated with high impedance amplifiers. Firstly the-high input resistance combined with the input capacitance of the transistor severely restricts the bandwidth available to process the incoming digital signal. The receiver acts as a leaky integrator and requires subsequent equalisation to restore bandwidth. Pulse shapes therefore become increasingly difficult to achieve as data rates increase when the equalisation reduces the output signal swing. This can lead to degradation of the signal-to-noise ratio on its output. Secondly the ramping effect caused by the integration of pulses introduces an AC dynamic range limitation when binary sequences are used which depends on the coding scheme used to construct the transmitted data sequence. The dynamic range is most severely restricted when long consecutive runs of one digital state are allowed. Thirdly, the mean photocurrent flows through an input bias resistor introducing a large voltage drop across the resistor which limits the DC dynamic range of the receiver. This is commonly alleviated by the introduction of a control loop which adjusts the voltage at the end of the resistor to compensate for the voltage dropped across it. However, the DC dynamic range is still limited by the power supply limitation of the control circuit. Reductions in the value of this resistor to increase the DC dynamic range decrease the sensitivity of the receiver.
One solution to the problem of extending the D.C. dynamic range of optical detectors has been proposed in an article by V. A. Druchevskii et al., "Extension of dynamic range of a photoreceiver based on a photodiode", in "INSTRUMENTS AND EXPERIMENTAL TECHNIQUES", vol. 23, no. 3, part 2, May-June 1980, pp. 758-760. The circuit described in this article increases the dynamic range of a photodiode detector by supplying the bias voltage to the photodiode from the output of an operational amplifier downstream of the photodiode, .the bias voltage being varied once the output voltage of the operational amplifier exceeds a preset threshold level.
One known solution of the problem of AC dynamic range limitation is to employ a differential code (dicode) scheme to encode the optically transmitted information to prevent ramping in the amplifier output. A means of achieving optical dicoding is described in the United Kingdom patent specification GB2,135,551. It comprises encoding a two-level, input electrical signal at a transmitter representative of digital data as a three level optical signal such that a transition in the input signal from low to high is encoded as a first light intensity level, a transition from high to low as a second intensity level, and an absence of a transition as a third intensity level intermediate the first and second light intensity levels.
A method of obtaining the electrical dicode of binary data to be used to modulate an optical source at the transmitter is to introduce a bit time delay and subtract the delayed electrical waveform from the undelayed electrical waveform. Since the three-level code is a differentiating code it may be directly decoded by the integrating behaviour of the high input impedance receiver. The three level code however imposes a linearity constraint on both the optical source and its drive circuitry as the three-level coder has to produce pulses which are symmetrical about the mid-level which itself should remain independent of the input binary state.